A serious problem encountered with all digital regulators is one of obtaining in a simple fashion those parameters which are to be used for tuning the regulator. Applicants' U.S. Pat. No. 4,549,123 describes a method which utilizes the fact that many processes experience periodic oscillation with relay feedback. A simple relay feedback experiment is carried out, and the amplitude and period of the resultant oscillation is used to tune a PID-type regulator (PID stands for Proportional, Integrated, Derivating) on the basis of the control concept according to Zeigler-Nichols-like design methods. This prior art tuning method is thus based solely on knowledge of the amplitude and period of the oscillation. Although this method is successful in the majority of cases, it is encumbered with the fundamental drawback that the model is based solely on one point in the frequency curve of the open loop system in a Nyquist diagram.
A process model can also be determined experimentally by first selecting a sampling period and introducing pertubation signals, whereafter the process mode is obtained with the aid of some suitable parameter assessment method. In those cases in which the regulator is tuned automatically, the oscillations are generated by conventional feedback and the parameters determined recursively and progressively, as further measurements become available.
The sampling period is a critical parameter, both in respect of conventional parameter estimation and of adaptive control. In order to estimate the sampling period, it is necessary to have prior knowledge of the time scale of the model and of the system engaged. This has long been the stumbling block in achieving automatic model construction and adaptive regulation or tuning.